Amplifier tube arrangement



June 7, 1938; K, WILHELM 2,119,694

AMPL I F IER TUBE ARRANGEMENT Filed Sept. 1, 1933 INVENTIOR KABL W/L ELMATTORNEY- Patented June 7, 1938 PATENT Fries AMPLIFIER, TUBE ARRANGEMENTKarl Wilhelm, Berlin, Germany, assignor to Telefnnken Gesellschaft fiirDrahtlose Telegraphic m. b. H., Berlin, Germany, a corporation ofGermany Application September 1, 1933, Serial No. 687,793

In Germany August 31, 1932 1 Claim.

This invention relates to amplifier tubes of the screen grid or pentodetype and more particularly with amplifiers whose last or power stageconsists of a screen grid tube or pentode tube. Usually when the signalvoltage applied to amplifier tubes is increased, there occurs a growthof the plate direct current. Such a case occurs when the grid is biasedslightly more than half Way between zero and cut-01f and the signalincreases to a point such that the negative peaks are beyond cut-ofi,while the positive peaks are insufiicient to cause grid current to flow.The

net result will be an increase in the average screen and plate currentsof the tube. Such a change in the plate current, especially in the powertube, causes a change in the load on the filter circuit or power pack,inasmuch as the power tube is usually connected direct to the power packand constitutes the main load thereof. In other words, increase of theoutput of the end or power tube, as a result of the change in voltage onthe output terminals of the power pack, reacts on the first stage of theamplifier since its plate voltage is also supplied from these sameterminals.

An object of this invention is to provide ways and means adapted torender the output direct current of an amplifier tube essentiallyindependent of large changes in grid voltage due to correspondingchanges in the signal voltage.

A second object of the invention is to provide a screen grid or pentodetube circuit arrangement in which the screen grid voltage isautomatically increased in case of a tube where the plate currentdecreases due to increase of the applied signal voltage, this increasein screen grid voltage being so controlled as to ofiset the decrease inthe direct plate current resulting from increase of signal voltage. Suchan operating condition is met with when the initial grid bias is sochosen that upon a large increase in signal voltage the grid is drivenpositive and rectification takes place in the grid-cathode circuit, withresulting direct current flowing through any resistance in the circuitbetween grid and cathode. This causes the average grid bias to becomemore negative thereby decreasing the average value of the plate current.Other objects of the invention will become apparent to those skilled inthe art as the description thereof proceeds.

Fora better understanding of the invention reference is made to theaccompanying drawing in which,

Fig. 1 shows a schematic arrangement of an amplifier embodying theinvention; and,

Fig. 2 shows a slightly modified arrangement.

Inasmuch as whenan increase of signal voltage is applied to a screengrid tube, the screen grid current of necessity increases simultaneouslywith the plate direct current, I have taken advantage of thisrelationship to influence the output current of the tube by means of thevariations in the screen grid current. According to this invention, thescreen-grid voltage is applied to the screen grid through a suitablydimensioned resistance.

As shown in the figures the screen grid vacuum tube I has a control gridl0, cathode H, screen grid l2 and plate 13. If desired a suppressor gridIt may also be used between grid l2 and the plate, as shown, but it willbe understood that the invention may be used with a tube not having asuppressor grid. As shown, the grid l4 may be electrically connected tocathode H within the tube. As also shown, signal voltage is applied tocontrol grid I0 through an audio transformer 2 whose primary may beconnected to a preceding audio amplifier tube l6 of a broadcast radioreceiver adapted to receive modulated radio signals. A load such as aloudspeaker l5 may be inserted in the plate circuit and a resistor 5 inseries in the screen grid circuit with the voltage source 4. If in thisarrangement the grid Ill is biased by any means (not shown to such anextent that the negative voltage peaks are cut off, While the positivepeaks are not suflicient to cause grid current to flow, the net resultwill be an increase in the average value of the plate and screencurrents. By a proper choice of the value of resistor 5, the screencurrent may be caused to decrease the screen voltage by the properamount so that the average current drawn from the source 4 by the tubewill remain substantially constant.

If it happens in any case that due to increase of applied signalvoltage, the direct plate current falls, the screen grid currentlikewise falling, there will be a lesser drop of voltage in resistor, 5with a consequent increase in voltage of grid l2. Such an operatingcondition will occur when the grid 16 is driven positive andrectification takes place in the grid-cathode circuit. The rectifiedcurrent flowing through any resistance in the grid circuit, such as thatof the transformer secondary winding, will cause the gridvoltage todecrease or become more negative, which causes 50 the average value ofthe screen and plate currents to decrease. In this system, the decreasedscreen current flowing through resistor 5 causes the screen voltage tobecome more positive; which will, in turn, cause the plate current toincrease to such an extent as to maintain a substantially constantaverage value in spite of the net increased negative grid bias due torectification. This increase of screen grid voltage will in turnincrease the steady plate current and by choosing resistor .5 of theproper value, the increase in plate current may be made substantially tocompensate for its decrease caused by signal voltage increase.

The power stage shown in Fig. 2 is arranged generally the same as inFig. 1 except that the proper screen grid voltage is secured from thecommon terminal of resistors 6 and T which are connected in seriesacross the plate battery 4 in voltage divider fashion. In this case, theresistors 6 and 1 are chosen of such values that the desiredcompensation of the direct plate current is secured, as above described.

Having described my invention, what I claim. and desire to secure byLetters Patent is:

A two stage audio frequency amplifier, each stage including anelectronic tube having a cathode, an anode and a grid electrode, anoutput circuit connected between the anode and cathode of the electronictube of the first stage, said output circuit including a transformerprimary and a source of positive potential in series, an input circuitconnecting the grid electrode and cathode of the electronic tube of thesecond stage, said input circuit including the secondary of saidtransformer, the electronic tube of said second stage being providedwith a screen grid located between its grid electrode and anode, anoutput circuit for said second electronic tube, said output circuitincluding a sound reproducer and said source of positive potentialconnected in series between the anode and cathode of the said secondelectronic tube, increases in the amplitude of audio frequency currentsin said transformer secondary winding acting to increase the averagevalue of the current passing through said sound reproducer whereby theload on said source of positive potential is varied to such an extentthat the positive potential applied to the anode of the electronic tubeof the first stage is varied in accordance therewith, and means forcompensating for said increase in current comprising a single resistorhaving one end directly connected to said screen grid and its other enddirectly connected to the positive terminal of said potential sourcewhereby said resistor is not traversed by the current flowing in theoutput circuit of either of said stages, said resistor being adapted tomaintain the load on said source substantially constant during operationof said amplifier. KARL WILHELM.

